The invention relates to a colour television display tube comprising an envelope having a neck, a cone, and a window, an electrode system provided in the neck to generate a number of electron beams, a display screen provided internally on the window and covered with an electrically conductive layer, and a shadow mask which is situated at a short distance from the display screen and which is electrically connected to the conductive layer provided on the display screen.
Such a colour television display tube is known, for example, from German Offenlegungsschrift No. 26 11 640. The resistance of the electric connection between the shadow mask and the display screen disclosed therein is made very small to maintain the shadow mask and the display screen at the same electrical potential during operation of the display tube.
As is known, an operating television receiver may be a source of interference for a radio receiver located nearly and tuned to a frequency in the long-wave or medium-wave band. This interference consists of electro-magnetic radiation in the frequency range from 150 kHz to approximately 1.5 MHz which originates from the video signal itself (video interference radiation) and from the deflection coils (deflection interference radiation). The video interference radiation results from the display screen being scanned with an electron beam modulated by the video signal. As a result of this the display screen potential fluctuates with the amplitude of the video signal, which fluctuations result in the emission of radiation in the above-mentioned frequency range by the display tube. The deflection interference radiation results inter alia from higher harmonics of the line flyback pulse being capacitively coupled to the conductive inner coating of the display tube and propagating via coupling capacities and resistances to the display screen and thence being radiated in the form of electro-magnetic energy.